The invention is directed to an optical waveguide fiber having reduced polarization mode dispersion (PMD), and a method for making such a waveguide fiber.
High performance telecommunications systems, i.e., those having transmission rates above about 5 Gb/s, or long regenerator spacing, with or without optical amplifiers, require waveguide fibers designed to limit all sources of signal distortion or signal power loss.
In particular, in high data rate transmission systems, essentially all sources of signal dispersion, including PMD, become potential data rate limiting factors, and must be controlled to enable such systems.
PMD may be controlled by controlling the birefringence of the waveguide fiber which causes the two polarization modes to propagate at different speeds in the waveguide fiber.
There will be no polarization mode dispersion if the waveguide fiber has perfect geometric symmetry and is free of stress which causes random birefringence.
However, it is impractical to pursue making a waveguide fiber essentially free of birefringence from the standpoint of cost and process control. Further, given a perfect glass waveguide fiber, birefringence, and thus PMD, can be induced in any of the several additional process steps required to put the glass waveguide into a usable form. Thus, birefringence can be induced in the coating, buffering, stranding or cabling process.
A practical alternative method for limiting PMD is to introduce birefringence into the waveguide fiber in a controlled way so that the polarization modes are mixed and therefore have travel times in the waveguide which may differ only slightly. An alternative statement is, the two polarization modes experience essentially little or no net birefringence over a pre-selected waveguide fiber length.
One approach to introducing birefringence into a waveguide fiber is discussed in "Applied Optics", Ashkin et al., Vol. 20 (13), page 2299. In that article a particular birefringence is impressed on the fiber by spinning the draw preform during draw.
In U.S. Pat. No. 5,298,047, Hart et al., a method for impressing a spin on the fiber during draw is disclosed. The number of required spins/meter is stated as being related to the beat length of the waveguide fiber.
It is noteworthy that both cited references teach that the periodicity of the spin must be less than the beat length of the wavelength.
The major drawbacks in a spinning technique during draw are:
at a reasonable draw speed the spinning rate is very high, thereby introducing perturbations into a drawing process which is already a complex and sensitive step in the waveguide manufacturing process; PA1 an additional draw control loop is required; and, PA1 waveguide costs are increased as percent good waveguide length decreases, at a step which occurs after significant investment in raw material and energy has already been made. PA1 substantially all of the perturbation must be impressed on the core of the waveguide fiber; and, PA1 the waveguide fiber must be drawn to an essentially uniform diameter.